The present invention relates generally to marine vessels for transporting liquefied gas and more particularly to structures for supporting liquefied gas tanks within the hulls of seagoing vessels.
Due to the rising cost of energy, the transoceanic shipment of liquefied gases has become practically and economically feasible. In particular, the shipment of liquefied natural gas, LNG, has received increasing interest as world energy supplies dwindle and costs increase.
For transoceanic shipment, gases are commonly condensed by cooling the gases to their boiling point and are transported as cryogenic liquids at atmospheric pressure and at temperatures below 50.degree. C., i.e., LNG liquefies at about 162.degree. C. at atmospheric pressure.
It has been found that spherical tanks are particularly suitable for the transoceanic transportation of liquefied gases due to the inherent structural strength of a sphere. For obvious safety reasons, the tanks must be securely attached to the hull of the ship or vessel. The securing of these tanks to the vessel should be accomplished with a minimum of heat transfer between the hull and the tank. The transfer of heat to a cryogenic tank, while it may be minimized, is inevitable and results in boiling off of some of the cryogenic liquid.
While boil-off natural gas may be recondensed by refrigeration units within the ship and/or used as a fuel to run the ships engines or equipment, any reduction in heat transfer to the tank results in increased energy efficiency and in more of the LNG reaching its destination.
A known method of attaching a spherical tank to the metal hull of the ship is by having a downwardly depending skirt piece from the tank abutting an upwardly extending matching skirt piece from the hull of the ship and welded thereto. As the body of the tank is covered by layers of insulating material, such as polyurethane foam, the abutting skirt pieces form the only direct thermal path between the surface of the tank and the hull of the ship. As the hull of the ship is generally at the temperature of the ocean water, which is well above the cryogenic temperature of the liquid in the tank, significant efficiencies may be achieved by minimizing heat transfer between the skirt pieces.
Typically, the tank will be formed of welded aluminum plates, and the skirt piece depending therefrom will similarly be made of aluminum. The upwardly extending skirt piece will typically be made of steel and attached to a steel hull. The different metals used in the skirt pieces necessitate a bimetallic weld which, particularly in view of the heat differentials between the hull and the cryogenic tank, has some inherent weaknesses and must be made with very careful quality control.
It is an object of the present invention to provide an improved structure for supporting a liquefied gas tank within a ship's hull. It is a further object to reduce heat transfer between the hull and the tank and to eliminate the need for a bimetallic weld to join a tank to the hull of the ship.